1. Field of the Invention
The invention relates to a radiation thermometer, which measures temperature of an object by detecting infrared energy radiated from the object.
2. Description of the Related Art
There has been, heretofore, a radiation thermometer (for example, refer to Japanese Patent Laid-Open No. 7-324981) which detects infrared energy radiated from an object to be measured and calibrates the infrared energy by emissivity of the object, hence to measure the actual temperature of the object to be measured.
Generally, a radiation thermometer detects infrared energy by heat sensitive element such as a thermopile. The thermopile includes an infrared radiation receiving portion (hot junction) having a plurality of thermocouples connected in a series. In this thermopile, when infrared radiation enters the infrared radiation receiving portion, there occurs a temperature difference between the hot junction and the cold junction depending on the amount of the incident infrared radiation, and thermo electromotive force is produced according to the difference. This thermo electromotive force corresponds to the temperature of the object to be measured.
The temperature of the cold junction of the thermopile changes according to the inner temperature of the thermopile. The temperature of the object measured by the thermopile is calibrated according to the inner temperature of the thermopile, while measuring the inner temperature of the thermopile with a thermistor. This can get an accurate temperature of the object to be measured.
In this radiation thermometer, since the infrared energy radiated from the object to be measured is invisible, a user cannot recognize which position of the object to be measured is being measured.
A radiation thermometer has been developed which can indicate a measuring position with light source such as laser diode (LD) or light emitting diode (LED) in order for a user to recognize the measuring position.
The radiation thermometer capable of indicating a measuring position, however, is apt to enlarge in size because it contains a light source such as a laser diode or a light emitting diode.
When the heat generated by the light source in order to indicate a measuring position is locally transmitted to the thermopile, the inner temperature of the thermopile becomes uneven. In this case, the thermistor cannot detect an accurate inner temperature of the thermopile.
Additionally, there is the case where the infrared radiation radiated from the peripheral members of the thermopile (for example, holding member of the thermopile and the like) enters the infrared radiation receiving portion. When the temperature of the peripheral members agrees with the inner temperature of the thermopile, the amount of the infrared radiation of the peripheral members entering the infrared radiation receiving portion can be calculated according to the inner temperature of the thermopile. By subtracting the infrared radiation amount of the peripheral members from all the infrared radiation amount entering the infrared radiation receiving portion, it is possible to accurately obtain the infrared radiation amount only for the object to be measured.
When the temperature of the peripheral members becomes uneven according to the heat of the light source, or when the temperature of the peripheral members is different from the inner temperature of the thermopile, the infrared radiation amount of the peripheral members entering the infrared radiation receiving portion cannot be calculated according to the inner temperature of the thermopile. It is impossible to accurately obtain the infrared radiation amount only for the object to be measured from all the infrared radiation entering the infrared radiation receiving portion.
In order to measure an accurate temperature of the object to be measured, it is necessary to set a distance between the thermopile and the light source as large as possible so as not to locally transmit heat to the thermopile and so as to make the atmospheric temperature near the thermopile equal to the inner temperature of the thermopile. As a result, downsizing of the radiation thermometer is increasingly difficult.
Since an output signal of the thermopile is very small, an output signal of the thermopile has to be amplified with a high amplification factor. When the output signal of the thermopile is affected by the noise, measurement accuracy is remarkably reduced.